1. Field of the Invention
This invention relates to the protection of multiple feeder lines in an electric power distribution system in which the feeder lines are each connected to the main bus through a feeder circuit breaker and in which any one of the feeder lines can be selectively alternatively connected to the main bus through a transfer bus protected by a single tie circuit breaker. More particularly, the invention relates to improved protection of the feeder lines when connected to the main bus through the transfer bus and tie circuit breaker.
2. Background Information
A common configuration for substations in an electric power distribution system comprises a number of feeder lines each connected to a main bus by its own circuit breaker. Each of the circuit breakers is controlled by a protective relay having a protection characteristic adapted to the characteristics of the associated feeder lines.
In order to permit a feeder circuit breaker to be taken out of service, such as for maintenance or replacement, while providing continued service to the feeder line, each of the feeder lines is connected to a common transfer bus by a disconnect switch. The transfer bus is connected to the main bus by a single tie circuit breaker. When a feeder circuit breaker is to be taken out of service, that circuit breaker is opened to disconnect the associated feeder line from the main bus. The associated disconnect switch is then closed to connect the affected feeder line to the transfer bus. When this connection has been made, the tie circuit breaker is closed to connect the feeder line to the main bus. The tie circuit breaker then provides protection for the feeder line until the feeder circuit breaker can be returned to service.
The tie circuit breaker is also controlled by a protective relay. The protective relay for the tie circuit breaker must have a protection characteristic which will adequately protect any of the feeder lines which may be connected to the transfer bus If the characteristics of all of the feeder lines do not differ too widely from one another, a single protection characteristic suitable for the longest feeder line can be used by the protective relay for the tie circuit breaker This will provide over protection for the other feeder lines, and hence could lead to false trips of the tie circuit breaker An alternative approach has been to dispatch a relay technician to manually change the settings of the tie circuit breaker protective relay to those of the protective relay for the feeder circuit breaker which has been taken out of service. This is a time consuming and expensive procedure which is subject to errors in the insertion of the new settings Another approach to accommodating for the different characteristics of the feeder lines which could be connected to the transfer bus has been to provide switches which scale the inputs to the protective relay for the tie circuit breaker to accommodate for variations in feeder line characteristics. Again, such a procedure requires a technician, is time consuming, expensive, and prone to errors.
The trend today is for the conventional electromechanical protective relays to be replaced by microprocessor based protective relays. Microprocessor based relays provide more options in establishing the protection characteristics of the protective relay and hence, provide a possibility of many more settings Such relays may in fact have several dozen settings. While many of these settings may be common for all of the feeder lines, there are still a great many settings that need to be made to adapt the protection characteristics of the tie circuit breaker protective relay to those of the protection relay on the circuit breaker which has been removed from service.
There is a need therefore for an improved scheme for protecting feeder lines when a dedicated feeder circuit breaker is removed from service.
There is a more particular need for an improved scheme for providing adequate protection without excessive overreach for any one of a number of feeder lines using a single tie circuit breaker controlled by a single protective relay.
There is an additional need for a scheme for adapting the protection characteristics of the protective relay of the single tie circuit breaker to the characteristics of the feeder line which the tie breaker connects to the main bus.
There is also need for such a scheme which is reliable, accurate and low in cost.
There is a further need for such a scheme which can be implemented automatically.
There is a still further need for such a scheme which provides default protection if indications are not clear which feeder line is connected to the transfer bus.